Control system for a two chamber gas discharge laser system

ABSTRACT

The present invention provides a control system for a modular high repetition rate two discharge chamber ultraviolet gas discharge laser. In preferred embodiments, the laser is a production line machine with a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. Feedback timing control techniques are provided for controlling the relative timing of the discharges in the two chambers with an accuracy in the range of about 2 to 5 billionths of a second even in burst mode operation. This MOPA system is capable of output pulse energies approximately double the comparable single chamber laser system with greatly improved beam quality.

The present invention is a continuation-in-part of Ser. No. 10/187,336filed Jun. 28, 2002 now U.S. Pat. No. 6,914,919, of Ser. No. 10/141,216filed May 7, 2002, now U.S. Pat. No. 6,693,939 of Ser. No. 10/036,676,filed Dec. 21, 2001, now U.S. Pat. No. 6,882,674 Ser. No. 10/036,727filed Dec. 21, 2001, now U.S. Pat. No. 6,865,210 Ser. No. 10/006,913filed Nov. 29, 2001, now U.S. Pat. No. 6,535,531 Ser. No. 10/000,991filed Nov. 14, 2001, now U.S. Pat. No. 6,795,474 Ser. No. 09/943,343,filed Aug. 29, 2001, now U.S. Pat. No. 6,567,450 Ser. No. 09/854,097,filed May 11, 2001, now U.S. Pat. No. 6,757,316 Ser. No. 09/848,043,filed May 3, 2001, now U.S. Pat. No. 6,549,551 Ser. No. 09/837,035 filedApr. 18, 2001 now U.S. Pat. No. 6,618,421 and Ser. No. 09/829,475 filedApr. 9, 2001, now U.S. Pat. No. 6,765,945 all of which are incorporatedherein by reference. This invention relates to lithography light sourcesfor integrated circuit manufacture and especially to gas discharge laserlithography light sources for integrated circuit manufacture.

BACKGROUND OF THE INVENTION Electric Discharge Gas Lasers

Electric discharge gas lasers are well known and have been availablesince soon after lasers were invented in the 1960s. A high voltagedischarge between two electrodes excites a laser gas to produce agaseous gain medium. A resonance cavity containing the gain mediumpermits stimulated amplification of light which is then extracted fromthe cavity in the form of a laser beam. Many of these electric dischargegas lasers are operated in a pulse mode.

Excimer Lasers

Excimer lasers are a particular type of electric discharge gas laser andthey have been known since the mid 1970s. A description of an excimerlaser, useful for integrated circuit lithography, is described in U.S.Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact ExcimerLaser.” This patent has been assigned to Applicants' employer, and thepatent is hereby incorporated herein by reference. The excimer laserdescribed in Patent '884 is a high repetition rate pulse laser. Theseexcimer lasers, when used for integrated circuit lithography, aretypically operated in an integrated circuit fabrication line“around-the-clock” producing many thousands of valuable integratedcircuits per hour; therefore, down-time can be very expensive. For thisreason most of the components are organized into modules which can bereplaced within a few minutes. Excimer lasers used for lithographytypically must have its output beam reduced in bandwidth to a fractionof a picometer. This “line-narrowing” is typically accomplished in aline narrowing module (called a “line narrowing package” or “LNP” forKrF and ArF lasers) which forms the back of the laser's resonant cavity(A line selection unit “LSU” is used for selecting a narrow spectralband in the F₂ laser.) The LNP is comprised of delicate optical elementsincluding prisms, a mirror and a grating. Electric discharge gas lasersof the type described in Patent '884 utilize an electric pulse powersystem to produce the electrical discharges, between the two elongatedelectrodes. In such prior art systems, a direct current power supplycharges a capacitor bank called a “charging capacitor” or “C₀” to apredetermined and controlled voltage called the “charging voltage” foreach pulse. The magnitude of this charging voltage may be in the rangeof about 500 to 1000 volts in these prior art units. After C₀ has beencharged to the predetermined voltage, a solid state switch is closedallowing the electrical energy stored on C₀ to ring very quickly througha series of magnetic compression circuits and a voltage transformer toproduce high voltage electrical potential in the range of about 16,000volts (or greater) across the electrodes which produce the dischargeswhich lasts about 20 to 50 ns.

Major Advances In Lithography Light Sources

Excimer lasers such as described in the '884 patent have during theperiod 1989 to 2001 become the primary light source for integratedcircuit lithography. More than 1000 of these lasers are currently in usein the most modem integrated circuit fabrication plants. Almost all ofthese lasers have the basic design features described in the '884patent. This is:

-   -   (1) a single, pulse power system for providing electrical pulses        across the electrodes at pulse rates of about 100 to 2500 pulses        per second;    -   (2) a single resonant cavity comprised of a partially reflecting        mirror-type output coupler and a line narrowing unit consisting        of a prism beam expander, a tuning mirror and a grating;    -   (3) a single discharge chamber containing a laser gas (either        krypton, fluorine and neon for KrF lasers or argon, fluorine and        neon for ArF lasers), two elongated electrodes and a tangential        fan for circulating the laser gas between the two electrodes        fast enough to clear the discharge region between pulses of        debris from the previous pulse, and    -   (4) a beam monitor for monitoring pulse energy, wavelength and        bandwidth of output pulses with a feedback control system for        controlling pulse energy, energy dose and wavelength on a        pulse-to-pulse basis.

During the 1989-2001 period, output power of these lasers has increasedgradually and beam quality specifications for pulse energy stability,wavelength stability and bandwidth have become increasingly tighter.Operating parameters for a popular lithography laser model used widelyin integrated circuit fabrication include pulse energy at 8 mJ, pulserate at 2,500 pulses per second (providing an average beam power of upto about 20 watts), bandwidth at about 0.5 pm full width half maximum(FWHM) and pulse energy stability at +/−0.35%.

Control of Pulse Energy and Dose Energy

When these gas discharge are used as light sources for integratedcircuit fabrication they are usually operated in what is known as “burstmode” operation. For example, a laser may be operated at a repetitionrate of 2,500 Hz for 0.3 seconds with pulse energies of about 8 mJ inorder to scan a die spot on a silicon wafer. The laser is then “off” fora period of about 0.3 seconds while the scanner positions the wafer andthe scanner optics for illumination of the next die spot. This routinecontinues until all of the die spots on the wafer (for example, 200 diespots) have been illuminated. Then the scanner equipment replaces thescanned wafer with another wafer. Thus, the typical laser operatingcycle would be:

-   (1) on 0.3 second-   (2) off 0.3 second-   (3) repeat steps (1) and (2) 200 times-   (4) off 10 seconds-   (5) repeat steps (1)-(4) continuously.

This type of operation may be continuous 24 hours per day, 7 days perweek with short down times for maintenance or other events.

It is very important that each die spot receive the desired quantity oflaser illumination and that the illumination be applied uniformly.Therefore the common practice is to monitor and control the pulse energyof each and every pulse to within a few percent (typically about 6percent) of a target value (for example, 8 mJ±0.5 mJ). Since there arethese variations in the pulse to pulse energies, a common practice is tomonitor the accumulated energy (referred to as dose energy) in a seriesof pulses (such as moving window of 30 pulses). These control techniquesrequire the monitoring of the pulse energy for every pulse, utilizationof information thus obtained to calculate control parameters forsubsequent pulses and the adjustment discharge voltages on a pulse topulse basis so that both pulse energy and dose are maintained withindesired ranges.

Monitoring and Control of Wavelength and Bandwidth

Modem integrated circuit fabrication requires the printing of circuitswith precise dimensions with accuracies in the range of about 0.5 to0.25 micron or less. This requires very precise focusing of the lightfrom the lithography light sources through projection optics of thestepper machines. Such precise focusing requires control of the centerwavelength and bandwidth of the light source. Therefore, the wavelengthsand bandwidth of the laser beam from there laser are typically monitoredfor each pulse and to assure that they remain within desired targetranges. Typically, the wavelength is controlled using a feedback controlbased on the monitored values of center wavelength. This feedback signalis used to position the pivoting mirror in the LNP described above tochange the direction at which laser light is reflected from defractiongrating also in the LNP. The centerline wavelength is monitored on apulse-to-pulse basis and the wavelength is feedback controlled on asclose to a pulse-to-pulse basis as feasible. The response time forcenter wavelength control of prior art lithographic lasers has been afew milliseconds. Bandwidth is monitored on a pulse-to-pulse basis.Bandwidth can be affected by F₂ concentration and gas pressure; so theseparameters are controlled to help assure that bandwidth values remainwithin desired ranges. Prior art lithography lasers typically do notprovide for fast response control of bandwidth.

Injection Seeding

A well-known technique for reducing the bandwidth of gas discharge lasersystems (including excimer laser systems) involves the injection of anarrow band “seed” beam into a gain medium. In some of these systems alaser producing the seed beam called a “master oscillator” is designedto provide a very narrow bandwidth beam in a first gain medium, and thatbeam is used as a seed beam in a second gain medium. If the second gainmedium functions as a power amplifier, the system is referred to as amaster oscillator, power amplifier (MOPA) system. If the second gainmedium itself has a resonance cavity (in which laser oscillations takeplace), the system is referred to as an injection seeded oscillator(ISO) system or a master oscillator, power oscillator (MOPO) system inwhich case the seed laser is called the master oscillator and thedownstream system is called the power oscillator. Laser systemscomprised of two separate systems tend to be substantially moreexpensive, larger and more complicated to build and operate thancomparable single chamber laser systems. Therefore, commercialapplication of these two chamber laser systems has been limited.

What is needed is a better control system for a pulse gas dischargelaser for operation at repetition rates in the range of about 4,000 to6,000 pulses per second or greater.

SUMMARY OF THE INVENTION

The present invention provides a control system for a modular highrepetition rate two discharge chamber ultraviolet gas discharge laser.In preferred embodiments, the laser is a production line machine with amaster oscillator producing a very narrow band seed beam which isamplified in the second discharge chamber. Feedback timing controltechniques are provided for controlling the relative timing of thedischarges in the two chambers with an accuracy in the range of about 2to 5 billionths of a second even in burst mode operation. This MOPAsystem is capable of output pulse energies approximately double thecomparable single chamber laser system with greatly improved beamquality.

In preferred embodiments a single very fast response resonant chargercharges in parallel (in less than 250 microseconds) separate chargingcapacitors for the master oscillator (MO) and the power amplifier (PA).Preferably the charger includes a De-Queuing circuit and a bleed downcircuit for precise control of charging voltage. In this embodiment afast response trigger timing module provides a trigger signal andmonitors light-out signals with better than nanosecond precision. In apreferred embodiment a control processor is programmed with an algorithmfor generating small charging voltage dithers to produce feedbackresponses from which trigger timing can be controlled to maintain laseroperation within desired ranges of laser efficiency and/or beam quality.In preferred embodiments the system may be operated as a KrF, an ArF oras an F₂ laser system. Pulse power components are preferably watercooled to minimize heating effects. The MO may be operated at a reducedgas pressure or lower F₂ concentration as compared to the PA fornarrower bandwidth. Also, the MO beam is apertured significantly toimprove beam spectral quality. Trigger timing techniques are alsodisclosed to produce improvements in beam quality. In addition, animproved line narrowing module also contributes to better beam spectralquality. In a described preferred embodiment, the laser system includesa control area network (CAN) with three CAN clusters providing two-waycommunication from a laser control platform to various laser modules.Preferred embodiment of the laser system also include a pulse stretcherfor increasing the length of laser pulses and a beam delivery unit withcontrol over beam alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a MOPA Laser System.

FIG. 1A is a cutaway drawing of the FIG. 1 System.

FIG. 1B is a drawing showing a mounting technique for laser components.

FIG. 1C is a block diagram showing a MOPA control system.

FIG. 1D is a block diagram of a portion of the control system.

FIG. 2 is a cross-section drawing of a laser chamber.

FIG. 3 is a schematic drawing showing features of a narrow band laseroscillator.

FIG. 3A is a drawing showing control features of a line narrowing unit.

FIG. 4 is a block diagram showing features of a pulse power controltechnique.

FIG. 4A shows the results of a trigger control technique.

FIG. 4B is a block diagram showing features of a control algorithm.

FIG. 4C shows response times of two similar laser units.

FIG. 5A is a circuit diagram of a pulse compression portion of a pulsepower system.

FIG. 5B is a block diagram-circuit diagram of a resonant charger system.

FIGS. 5C1, 5C2 and 5C3 show features of a MOPA trigger controltechnique.

FIGS. 6A1 and 6A2 show various power amplifier configurations andresults.

FIG. 6B shows the relationship between PA input and PA output.

FIGS. 6C, 6D, 6E and 6F shows the effect of variations in time delaybetween MO discharge and PA discharge.

FIG. 6F1 shows the time delay graphically.

FIG. 6G shows elements of an energy control technique.

FIG. 6H shows a trigger control technique.

FIG. 6I illustrates a feedback timing control technique.

FIG. 6J shows the relationship between the voltage on Cp and light outin a MOPA system.

FIG. 6K shows the effect of inductor temperature on timing.

FIGS. 7, 7A, 8, 9, 9A and 9B illustrate pulse power components andtechniques to cool them.

FIGS. 10 and 10A show features of a pulse transformer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS MOPA LASER LITHOGRAPHYLIGHT SOURCE General Description

A laser system incorporating a first preferred embodiment of the presentinvention is shown in FIG. 1. In this embodiment a 193 nm ultravioletlaser beam is provided at the input port of a lithography machine 2 suchas stepper or scanner machines supplied by Canon or Nikon withfacilities in Japan or ASML with facilities in the Netherlands. Thislaser system includes a laser energy control system for controlling bothpulse energy and accumulated dose energy output of the system at pulserepetition rates of 4,000 Hz or greater. The system provides extremelyaccurate triggering of the discharges in the two laser chambers relativeto each other with both feedback and feed-forward control of the pulseand dose energy.

In this case the main components of the laser system 4 are installedbelow the deck on which the scanner is installed. However, this lasersystem includes a beam delivery unit 6, which provides an enclosed beampath for delivering the laser beam to the input port of scanner 2. Thisparticular light source system includes a master oscillator 10 and apower amplifier 12 and is a type of laser system known as MOPA system.The light source also includes a pulse stretcher. This light sourcerepresents an important advancement in integrated circuit light sourcesover the prior art technique of using a single laser oscillator toprovide the laser light.

The master oscillator and the power amplifier each comprise a dischargechamber similar to the discharge chamber of prior art single chamberlithography laser systems. These chambers (described in detail below)contain two elongated electrodes, a laser gas, a tangential forcirculating the gas between the electrodes and water-cooled finned heatexchangers. The master oscillator produces a first laser beam 14A whichis amplified by two passes through the power amplifier to produce laserbeam 14B as shown in FIG. 1. The master oscillator 10 comprises aresonant cavity formed by output coupler 10C and line narrowing package10B both of which are described generally in the background section andin more detail below in the referenced patents and parent applications.The gain medium for master oscillator 10 is produced between two 50-cmlong electrodes contained within master oscillator discharge chamber10A. Power amplifier 12 is basically a discharge chamber and in thispreferred embodiment is almost exactly the same as the master oscillatordischarge chamber 10A providing a gain medium between two elongatedelectrodes but power amplifier 12 has no resonant cavity. This MOPAconfiguration permits the master oscillator to be designed and operatedto maximize beam quality parameters such as wavelength stability andvery narrow bandwidth; whereas the power amplifier is designed andoperated to maximize power output. For example, the current state of theart light source available from Cymer, Inc. (Applicants' employer) is asingle chamber 5 mJ per pulse, 4 kHz, ArF laser system. The system shownin FIG. 1 is a 10 mJ per pulse (or more, if desired) 4 kHz ArF lasersystem producing at least twice the average ultraviolet power withsubstantial improvement in beam quality. For this reason the MOPA systemrepresents a much higher quality and much higher power laser lightsource. FIG. 1A shows the general location of the above referred tocomponents in one version of the MOPA modular laser system.

The Master Oscillator

The master oscillator 10 shown in FIGS. 1 and 1A is in many ways similarto prior art ArF lasers such as described in the '884 patent and in U.S.Pat. No. 6,128,323 and has many of the features of the ArF laserdescribed in U.S. patent application Ser. No. 09/854,097 except theoutput pulse energy is typically about 0.1 mJ instead of about 5 mJ. Asdescribed in great detail in the '097 application, major improvementsover the '323 laser are provided to permit operation at 4000 Hz andgreater. The master oscillator of the present invention is optimized forspectral performance including precise wavelengths and bandwidthcontrol. This result is a much more narrow bandwidth and improvedwavelength stability and bandwidth stability. The master oscillatorcomprises discharge chamber 10A as shown in FIG. 1, FIG. 1A, and FIG. 2in which are located a pair of elongated electrodes 110A2 and 110A4,each about 50 cm long and spaced apart by about 0.5 inch. Anode 10A4 ismounted on flow shaping anode support bar 10A6. Four separate finnedwater-cooled heat exchanger units 10A8 are provided. A tangential fan10A10 is driven by two motors (not shown) for providing a laser gas flowat velocities of up to about 80 m/s between the electrodes. The chamberincludes window units (not shown) with CaF₂ windows positioned at about45° with the laser beam. An electrostatic filter unit having an intakeat the center of the chamber, filters a small portion of the gas flow asindicated at 11 in FIG. 2 and the cleaned gas is directed into each ofthe window units in the manner described in U.S. Pat. No. 5,359,620(incorporated herein by reference) to keep discharge debris away fromthe windows. The gain region of the master oscillator is created bydischarges between the electrodes through the laser gas which in thisembodiment is comprised of about 3% argon, 0.1% F₂ and the rest neon.The gas flow clears the debris of each discharge from the dischargeregion prior to the next pulse. The resonant cavity is created at theoutput side of the oscillator by an output coupler 10C (as shown inFIG. 1) which is comprised of a CaF₂ mirror mounted perpendicular to thebeam direction and coated to reflect about 30% of light at 193 nm and topass about 70% of the 193 nm light. The opposite boundary of theresonant cavity is a line narrowing unit 10B as shown in FIG. 1 similarto prior art line narrowing units described in U.S. Pat. No. 6,128,323.Important improvements in this line narrowing package as shown in FIG. 3include four CaF beam expanding prisms 112a-d for expanding the beam inthe horizontal direction by 45 times and a tuning mirror 114 controlledby a stepper motor for relatively large pivots and a piezoelectricdriver for providing extremely fine tuning of the center linewavelength. FIG. 3A shows the stepper motor 82 and piezoelectric driver83. The stepper motor provides its force to mirror 114 through lever arm84 and piezoelectric driver 83 applies its force on the fulcrum 85 ofthe lever system. An LNP processor 89 located at the LNP controls boththe stepper motor and the piezoelectric driver based on feedbackinstructions from a line center analysis module (LAM) 7. Echelle grating10C3 having about 80 facets per mm is mounted in the Litrowconfiguration and reflects a very narrow band of UV light selected fromthe approximately 300 pm wide ArF natural spectrum. This line narrowingunit is preferably purged continuously during operation with helium.(Nitrogen is another alternate purge gas.) Preferably the masteroscillator is operated at a much lower F₂ concentration than istypically used in prior art lithography light sources. This results insubstantial reductions in the bandwidth since Applicants have shown thatbandwidth decreases substantively with decreasing F₂ concentrations.Another important improvement is a narrow rear aperture which limits thecross section of the oscillator beam to 1.1 mm in the horizontaldirection and 7 mm in the vertical direction. Control of the oscillatorbeam is discussed below.

In preferred embodiments the main charging capacitor banks for both themaster oscillator and the power amplifier are charged in parallel so asto reduce jitter problems. This is desirable because the times for pulsecompression in the pulse compression circuits of each of the two pulsepower systems is very dependent on the level of the charge of thecharging capacitors. Preferably pulse energy output is controlled on apulse-to-pulse basis by adjustment of the charging voltage. This limitsthe use of voltage to control beam parameters of the master oscillator.However, laser gas pressure and F₂ concentration can be easilycontrolled separately in each of the two chambers to achieve desirablebeam parameters over a wide range of pulse energy levels and laser gaspressures. Bandwidth decreases with decreasing F₂ concentration andlaser gas pressure. These control features are in addition to the LNPcontrols which are discussed in detail below.

Power Amplifier

The power amplifier in this preferred embodiment is comprised of a laserchamber which, with its internal components, as stated above is verysimilar to the corresponding master oscillator discharge chamber. Havingthe two separate chambers allows the pulse energy and dose energy (i.e.,integrated energy in a series of pulses) to be controlled, to a largeextent, separately from wavelength and bandwidth. This permits higherpower and better dose stability. All of the components of the chamberare the same and are interchangeable during the manufacturing process.However, in operation, the gas pressure can be substantially higher inthe PA as compared to the MO. Laser efficiency increases with F₂concentration and laser gas pressure over a wide range of F₂concentration; however lower F₂ concentrations can result in smallerbandwidths. The compression head 12B of the power amplifier is alsosubstantially identical in this embodiment to the 10B compression headof the MO and the components of the compression heads are alsointerchangeable during manufacture. This close identity of the chambersand the electrical components of the pulse power systems helps assurethat the timing characteristics of the pulse forming circuits are thesame or substantially the same so that jitter problems are minimized.One minor difference is that the capacitors of the MO compression headcapacitor bank are more widely positioned to produce a substantiallyhigher inductance as compared to the PA.

The power amplifier is configured for two beam passages through thedischarge region of the power amplifier discharge chamber as shown inFIG. 1. The beam oscillates several times through the chamber 10Abetween LNP 10B and output coupler 10C (with 30 percent reflectance) ofthe MO 10 as shown in FIG. 1 and is severely line narrowed on itspassages through LNP 10C. The line narrowed seed beam is reflecteddownward by a mirror in the MO wavelength engineering box (MO WEB) 24and reflected horizontally at an angle slightly skewed (with respect tothe electrodes orientation) through chamber 12. At the hack end of thepower amplifier beam reverser 28 reflects the beam back for a secondpass through PA chamber 12 horizontally in line with the electrodesorientation.

The charging voltages preferably are selected on a pulse-to-pulse basisto maintain desired pulse and dose energies. F₂ concentration and lasergas pressure can be adjusted to provide a desired operating range ofcharging voltage (since as stated above charging voltage decreases withincreasing F₂ concentration and gas pressure for a given output pulseenergy). This desired range can be selected to produce a desired valueof dE/dV since the change in energy with voltage is also a function ofF₂ concentration and laser gas pressure. F2 gas is depleted in thechambers over time and their depletion is in general accommodated by acorresponding increase in charging voltage to maintain desired pulseenergy. The timing of injections is preferable based on chargingvoltage. The frequency of injections preferably is kept high (and theinserted quantity is preferably kept small) to keep conditionsrelatively constant and injections can be continuous or nearlycontinuous. However, some users of these laser systems may prefer largerdurations (such as 2 hours) between F₂ injections. Some users may preferthat the laser be programmed to not fire during F₂ injections.

MOPA Control System

FIG. 1C is a block diagram showing many of the important controlfeatures of a preferred embodiment of the present invention. The controlsystem includes RS232 laser-scanner interface hardware 600 whichcomprises special software permitting laser control from any of severaltypes of lithography machines 2 (which could be a stepper or scannermachine) or a laser operation control paddle 602. Central processingunit 604 is the master control for the MOPA system and receivesinstructions through four serial ports 606 and interface hardware 600,from lithography machine 2 and operator control paddle 602.

Laser control CPU 604 communicates to fire control CPU 608 throughcommunication PCI buses 610, 612, and 614. Fire control platform CPU 608controls the charging of the charging capacitors in both the MO and thePA which are charged in parallel by resonant charger 49. Fire controlCPU 608 sets the HV target for each pulse and provides the trigger tobegin charging. (This CPU also implements timing control and energycontrol algorithms discussed in more detail below). A timing energymodule 618 receives signals from light detectors in MO and PA photodetector modules 620 and 622 and based on these signals and instructionsfrom command module 616 provides feedback trigger signals to MOcommutator 50A and PA commutator 50B which triggers switches initiatingdischarges from the MO and PA charging capacitors 42 as shown in FIG. 5and initiates the pulse compressions resulting in the generation ofdischarge voltage in the peaking capacitors 82 to produce discharges inthe MO and the PA. Additional details of the TEM are shown in FIG. 1D.

The preferred timing process is as follows: command module 616 sendstrigger instructions to timing energy module 618 27 microseconds priorto a desired light out (i.e., time of first edge of laser pulse)providing the precise times for triggering switches 46 in both the MOand the PA. The TEM synchronizes timing signals with its internal clockby establishing a reference time called the “TEM reference” and thencorrelates trigger and light out signals to that reference time. The TEMthen issues trigger signals to MO switch 46 in the MO commutator 50Awith an accuracy better than about 250 picoseconds and a few ns later(in accordance with the instructions from command module 616) issues atrigger signal to the PA switch 46 in the PA commutator 50B also with anaccuracy better than about 250 ps. The TEM then monitors the time oflight out signals from PD modules 620 and 622 with an accuracy betterthan about 250 ps relative to the TEM reference time. These time dataare then transmitted by the TEM 618 to command module 616 which analyzesthese data and calculates the proper timing (based on algorithmsdiscussed below) for the next pulse and 27 microseconds prior to thenext pulse, command module 616 sends new trigger instructions to timingenergy module 618. (The TEM can also monitor voltage on peakingcapacitor banks 82 and feedback trigger control can also be based on thetime voltages on the peaking capacitor banks cross a specifiedthreshold.)

Thus, the discharge timing job is shared between TEM module 618 andcommand module 616. Communication between the two modules is along 10megabit synchronous serial link shown at 617 in FIG. 1C. Module 618provide extremely fast trigger generation and timing methodology andmodule 616 provides extremely fast calculations. Both working togetherare able to monitor timing, provide feedback, calculate the next timingsignal using a complicated algorithm and provide two trigger signals tothe commutators all within time windows of less than 250 microsecondsand to assure relative triggering accuracy of the two discharges of lessthan about 2 to 5 billionths of a second! TEM module also provide alight out signal to stepper/scanner 2. This triggering process can bemodified by instructions from the stepper/scanner 2 or by the laseroperator through user interface paddle 602. High speed monitoring andtrigger circuits of the type used in TEM module are available fromsuppliers such as Highland Technologies with offices in San Francisco;Berkley Nucleonics with offices in San Rafael, Calif., AndersonEngineering with offices in San Diego, Calif. and Stanford Research withoffices in Pasadena, Calif. The importance of the accuracy of thesetiming circuits and issues and features relating to these triggercircuits are discussed in more detail below.

Timing modules like the TEM require sub-nanosecond time resolution. Inpreferred embodiments, Applicants employ a trick to achieve much betterthan 1-nanosecond resolution (i.e., about 100 ps resolution) with a widedynamic range using a digital counter such as a 20 or 40 MHz crystaloscillator. The crystal oscillator provides clock signals at 25 or 50 nsintervals but these signals are utilized to charge a very linear analogcapacitive charging circuit. The voltage on the capacitor is then readto determine time with sub-nanosecond accuracy.

Wavelength control is provided by LNM controller 624 with instructionsfrom fire control command module 616 based on feedback signals from linecenter analysis module (LAM) 7 which monitors the output of the MO.Preferred techniques for measuring the line center wavelength arediscussed below.

Control of other elements of the laser system is provided by a controlarea network (CAN) as indicated on FIG. 1C. CAN interface 626 interfaceswith laser control platform 604 and provides control information tothree CAN clusters: power cluster 628, left optics bay cluster 630, andright optics bay cluster 632. This CAN network provides two-waycommunication with these modules providing control from laser controlplatform 604 to the various modules and providing operational data fromthe modules back to the laser control platform.

Data acquirization can be provided through switch 636 and Cymer-on-Linemodule 634 which can collect and store huge amounts of data and make itavailable through Internet systems all as described in U.S. patentapplication Ser. No. 09/733,194, which is incorporated by referenceherein. Field services port 638 provides access to CPU 608 and CPU 604for special analysis and tests. Also eight BNC connectors 640 areavailable through digital-to-analog converter 642 for special monitors.

Test Results

Applicants have conducted extensive testing of the basic MOPAconfiguration shown in FIG. 1 with various optical paths as shown inFIG. 6A1. The results are shown in 6A2. Designs that have been testedinclude single pass, straight double pass, single pass with dividedamplifier electrodes and tilted double pass. FIG. 6B shows system outputpulse energy as a function of PA input energy for the skewed double passconfiguration at charging voltage ranging from 650 V to 1100 V. FIG. 6Cshows the shape of the output pulse as a function of time delay betweenbeginning of the oscillator and the amplifier pulses for four inputenergies. FIG. 6D shows the effect of time delay between pulses onoutput beam bandwidth. This graph also shows the effect of delay onoutput pulse energy. This graph shows that bandwidth can be reduced atthe expense of pulse energy by increasing the delay. FIG. 6E shows thatthe laser system pulse duration can also be extended somewhat at theexpense of pulse energy.

PULSE POWER SYSTEM Pulse Power Circuit

In the preferred embodiment shown in FIG. 1 the basic pulse powercircuits for both the MO and the PA are similar to pulse power circuitsof prior art excimer laser light sources for lithography. Separate pulsepower circuits downstream of the charging capacitors are provided foreach discharge chamber. Preferably a single resonant charger charges twocharging capacitor banks connected in parallel to assure that bothcharging capacitor banks are charged to precisely the same voltage. Thispreferred configuration is shown in FIG. 4 and FIG. 5C1. FIG. 5A showsimportant elements of a the basic pulse compression circuit which isused for both the MO and the PA. FIG. 5C2 shows a simplified version ofthis circuit.

Resonant Charger

A preferred resonant charger system 49 is shown in FIG. 5B. Theprincipal circuit elements are:

-   -   I1—A three-phase power supply 300 with a constant DC current        output.    -   C-1—A source capacitor 302 that is an order of magnitude or more        larger than C₀ capacitor banks 42. There are two of the        capacitor banks CO₁₀ and CO₁₂ which are charged in parallel.    -   Q1, Q2, and Q3—Switches to control current flow for charging and        maintaining a regulated voltage on C₀ capacitor banks.    -   D1, D2, and D3—Provides current single direction flow.    -   R1, and R2—Provides voltage feedback to the control circuitry.    -   R3—Allows for rapid discharge of the voltage on C₀ in the event        of a small over charge.    -   L1—Resonant inductor between C-1 capacitor 302 and C₀ capacitor        banks 42 to limit current flow and setup charge transfer timing.    -   Control Board 304—Commands Q1, Q2, and Q3 open and closed based        upon circuit feedback parameters.

This circuit includes switch Q2 and diode D3, together known as aDe-Qing switch. This switch improves the regulation of the circuit byallowing the control unit to short out the inductor during the resonantcharging process. This “de-qing” prevents additional energy stored inthe current of the charging inductor, L1, from being transferred to theC₀ capacitor banks.

Prior to the need for a laser pulse the voltage on C-1 is charged to600-800 volts and switches Q1-Q3 are open. Upon command from the laser,Q1 closes. At this time current would flow from C-1 to C₀ through thecharge inductor L1. As described in the previous section, a calculatoron the control board evaluates the voltage on C₀ and the current flowingin L1 relative to a command voltage set point from the laser. Q1 openswhen the voltage on the CO capacitor banks plus the equivalent energystored in inductor L1 equals the desired command voltage. Thecalculation is:V_(f)=[V_(COs) ²+((L₁*I_(LIs) ²)/C_(O))]^(0.5)Where:

-   V_(f)=The voltage on C₀ after Q1 opens and the current in L1 goes to    zero.-   V_(COs)=The voltage on C₀ when Q1 opens.-   I_(L1s)=The current flowing through L₁ when Q1 opens.

After Q1 opens the energy stored in L1 starts transferring to the COcapacitor banks through D2 until the voltage on the CO capacitor banksapproximately equals the command voltage. At this time Q2 closes andcurrent stops flowing to CO and is directed through D3. In addition tothe “de-qing” circuit, Q3 and R3 form a bleed-down circuit which allowsadditional fine regulation of the voltage on CO.

Switch Q3 of bleed down circuit 216 will be commanded closed by thecontrol board when current flowing through inductor L1 stops and thevoltage on C₀ will be bled down to the desired control voltage; thenswitch Q3 is opened. The time constant of capacitor C_(o) and resistorR3 should be sufficiently fast to bleed down capacitor C_(o) to thecommand voltage without being an appreciable amount of the total chargecycle.

As a result, the resonant charger can be configured with three levels ofregulation control. Somewhat crude regulation is provided by the energycalculator and the opening of switch Q1 during the charging cycle. Asthe voltage on the CO capacitor banks nears the target value, thede-qing switch is closed, stopping the resonant charging when thevoltage on C_(o) is at or slightly above the target value. In apreferred embodiment, the switch Q1 and the de-qing switch is used toprovide regulation with accuracy better than +/−0.1%. If additionalregulation is required, the third control over the voltage regulationcould be utilized. This is the bleed-down circuit of switch Q3 and R3(shown at 216 in FIG. 5B) to discharge the CO's down to the precisetarget value.

Improvements Downstream of the CO's

As indicated above, the pulse power system of the MO and the PA of theirpreferred embodiment each utilizes the same basic design (FIG. 5A) aswas used in single chamber systems as described in U.S. application Ser.No. 10/036/676. Important advancements described and claimed hereinrelate to the combining of these two separate pulse power system toassure efficient laser operation with precise timing control and precisecontrol of laser beam quality. In addition, some significantimprovements which were described in the above parent applications wererequired for the approximate factor of 3 increase in heat load resultingfrom the greatly increased repetition rate as compared to prior artlithographic laser systems. These improvements are discussed below.

Detailed Commutator and Compression Head Description

In this section, we describe details of fabrication of the commutatorand the compression head.

Solid State Switch

Solid state switch 46 is an P/N CM 800 HA-34H IGBT switch provided byPowerex, Inc. with offices in Youngwood, Pa. In a preferred embodiment,two such switches are used in parallel.

Inductors

Inductors 48, 54 and 64 are saturable inductors similar to those used inprior systems as described in U.S. Pat. Nos. 5,448,580 and 5,315,611.

FIG. 7 shows a preferred design of the L_(o) inductor 48. In thisinductor tour conductors from the two IGBT switches 46B pass throughsixteen ferrite toroids 49 to form part 48A an 8 inch long hollowcylinder of very high permeability material with an ID of about 1 inchand an OD of about 1.5 inch. Each of the four conductors are thenwrapped twice around an insulating doughnut shaped core to form part48B. The four conductors then connect to a plate which is in turnconnected to the high voltage side of the C₁ capacitor bank 52.

A preferred sketch of saturable inductor 54A is shown in FIG. 8. In thiscase, the inductor is a single turn geometry where the assembly top andbottom lids 541 and 542 and center mandrel 543, all at high voltage,form the single turn through the inductor magnetic cores. The outerhousing 54A1 is at ground potential. The magnetic cores are 0.0005″thick tape wound 50-50% Ni—Fe alloy provided by Magnetics of Butler, Pa.or National Arnold of Adelanto, Calif. In addition, a ceramic disk (notshown) is mounted underneath the reactor bottom lid to help transferheat from the center section of the assembly to the module chassis baseplate. FIG. 8 also shows the high voltage connections to one of thecapacitors of the C₁ capacitor bank 52 and to a high voltage lead on oneof the induction units of the 1:25 step up pulse transformer 56. Thehousing 545 is connected to the ground lead of unit 56.

This inductor is cooled by a water cooled jacket 54A1. The cooling line54A2 is routed within the module to wrap around jacket 54A1 and throughaluminum base plate where the IGBT switches and Series diodes aremounted. These three components make up the majority of the powerdissipation within the module. Other items that also dissipate heat(snubber diodes and resistors, capacitors, etc.) are cooled by forcedair provided by the two fans in the rear of the module.

Since the jacket 54A1 is held at ground potential, there are no voltageisolation issues in directly attaching the cooling tubing to the reactorhousing. This is done by press-fitting the tubing into a dovetail groovecut in the outside of the housing as shown at 54A3 and using a thermallyconductive compound to aid in making good thermal contact between thecooling tubing and the housing.

The water-cooled compression head is similar in the electrical design toa prior art air-cooled version (the same type ceramic capacitors areused and similar material is used in the reactor designs). The primarydifferences in this case are that the module must run at higherrep-rates and therefore, higher average power. In the case of thecompression head module, the majority of the heat is dissipated withinthe modified saturable inductor 64A. Cooling the subassembly is not asimple matter since the entire housing operates with short pulses ofvery high voltages. The solution to this issue as shown in FIGS. 9, 9Aand 9B is to inductively isolate the housing from ground potential. Thisinductance is provided by wrapping the cooling tubing around twocylindrical forms that contain a ferrite magnetic core. Both the inputand output cooling lines are coiled around cylindrical portions of aferrite core formed of the two cylindrical portions and the two ferriteblocks as shown in FIGS. 9, 9A and 9B.

The ferrite pieces are made from CN-20 material manufactured by CeramicMagnetics, Inc. of Fairfield, N.J. A single piece of copper tubing(0.187″ diameter) is press fit and wound onto one winding form, aroundthe housing 64A1 of inductor 64A and around the second winding form.Sufficient length is left at the ends to extend through fittings in thecompression head sheet metal cover such that no cooling tubing jointsexist within the chassis.

The inductor 64A comprises a dovetail groove as shown at 64A2 similar tothat used in the water-cooled commutator first stage reactor housing.This housing is much the same as previous air-cooled versions with theexception of the dovetail groove. The copper cooling-water tubing ispress fit into this groove in order to make a good thermal connectionbetween the housing and the cooling-water tubing. Thermally conductivecompound is also added to minimize the thermal impedance. Inductor 64Aprovides two loops around magnetic core 64A3 which is comprised of fourcoils of tape.

Bias current as shown in FIG. 5A is supplied by a dc-dc converter in thecommutator through a cable into the compression head. The current passesthrough the “positive” bias inductor L_(B2) and is connected to the Cp−1voltage node. The current then splits with a portion returning to thecommutator through the HV cable (passing through the transformersecondary to ground and back to the dc-dc converter). The other portionpasses through the compression head reactor Lp−1 (to bias the magneticswitch) and then through the cooling-water tubing “negative” biasinductor L_(B3) and back to ground and the dc-dc converter. By balancingthe resistance in each leg, the designer is able to ensure thatsufficient bias current is available for both the compression headreactor and the commutator transformer.

The “positive” bias inductor L_(B2) is made very similarly to the“negative” bias inductor L_(B3). In this case, the same ferrite bars andblocks are used as a magnetic core. However, two 0.125″ thick plasticspacers are used to create an air gap in the magnetic circuit so thatthe cores do not saturate with the dc current. Instead of winding theinductor with cooling-water tubing, 18 AWG teflon wire is wound aroundthe forms.

Cooling Other High Voltage Components

Although the IGBT switches “float” at high voltage, they are mounted onan aluminum base electrically isolated from the switches by a 1/16 inchthick alumina plate. The aluminum base plate which functions as a heatsink and operates at ground potential and is much easier to cool sincehigh voltage isolation is not required in the cooling circuit. A drawingof a water cooled aluminum base plate is shown in FIG. 7A. In this case,the cooling tubing is pressed into a groove in an aluminum base on whichthe IGBT's are mounted. As with the inductor 54a, thermally conductivecompound is used to improve the overall joint between the tubing and thebase plate.

The series diodes also “float” at high potential during normaloperation. In this case, the diode housing typically used in the designprovides no high voltage isolation. To provide this necessaryinsulation, the diode “hockey puck” package is clamped within a heatsink assembly which is then mounted on top of a ceramic base that isthen mounted on top of the water-cooled aluminum base plate. The ceramicbase is just thick enough to provide the necessary electrical isolationbut not too thick to incur more than necessary thermal impedance. Forthis specific design, the ceramic is 1/16 thick alumina although othermore exotic materials, such as beryllia, can also be used to furtherreduce the thermal impedance between the diode junction and the coolingwater.

A second embodiment of a water cooled commutator utilizes a single coldplate assembly which is attached to the chassis baseplate for the IGBT'sand the diodes. The cold plate may be fabricated by brazing single piecenickel tubing to two aluminum “top” and “bottom” plates. As describedabove, the IGBT's and diodes are designed to transfer their heat intothe cold plate by use of the previously mentioned ceramic disksunderneath the assembly. In a preferred embodiment of this invention,the cold plate cooling method is also used to cool the IGBT and thediodes in the resonant charger. Thermally conductive rods or a heat pipecan also be used to transfer heat from the outside housing to thechassis plate.

In prior art pulse power systems, oil leakage from electrical componentshas been a potential problem. In this preferred embodiment, oilinsulated components are limited to the saturable inductors.Furthermore, the saturable inductor 64 as shown in FIG. 9 is housed in apot type oil containing housing in which all seal connections arelocated above the oil level to substantially eliminate the possibilityof oil leakage. For example, the lowest seal in inductor 64 is shown at308 in FIG. 9.

Capacitors

Capacitor banks 42, 52, 62 and 82 (i.e., C_(o), C₁, C_(p−1) and C_(p))as shown in FIG. 5A are all comprised of banks of off-the-shelfcapacitors connected in parallel. Capacitors 42 and 52 are film typecapacitors available from suppliers such as Vishay Roederstein withoffices in Statesville, N.C. and Wima of Germany. Applicants' preferredmethod of connecting the capacitors and inductors is to solder them topositive and negative terminals on special printed circuit board havingheavy nickel coated copper leads in a manner similar to that describedin U.S. Pat. No. 5,448,580. Capacitor banks 62 and 64 are typicallycomprised of a parallel array of high voltage ceramic capacitors fromvendors such as Murata or TDK, both of Japan. In a preferred embodimentfor use on this ArF laser, capacitor bank 82 (i.e., C_(p)) comprised ofa bank of thirty three 0.3 nF capacitors for a capacitance of 9.9 nF;C_(p−1) is comprised of a bank of twenty four 0.40 nF capacitors for atotal capacitance of 9.6 nF; C₁ is a 5.7 μF capacitor bank and C_(o) isa 5.3 μF capacitor bank.

Pulse Transformer

Pulse transformer 56 is also similar to the pulse transformer describedin U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulsetransformers of the present embodiment has only a single turn in thesecondary winding and 24 induction units equivalent to 1/24 of a singleprimary turn for an equivalent step-up ratio of 1:24. A drawing of pulsetransformer 56 is shown in FIG. 10. Each of the 24 induction unitscomprise an aluminum spool 56A having two flanges (each with a flat edgewith threaded bolt holes) which are bolted to positive and negativeterminals on printed circuit board 56B as shown along the bottom edge ofFIG. 10. (The negative terminals are the high voltage terminals of thetwenty four primary windings.) Insulators 56C separate the positiveterminal of each spool from the negative terminal of the adjacent spool.Between the flanges of the spool is a hollow cylinder 1 1/16 inches longwith a 0.875 OD with a wall thickness of about 1/32 inch. The spool iswrapped with one inch wide, 0.7 mil thick Metglas™ 2605 S3A and a 0.1mil thick mylar film until the OD of the insulated Metglas™ wrapping is2.24 inches. A prospective view of a single wrapped spool forming oneprimary winding is shown in FIG. 10A.

The secondary of the transformer is a single stainless steel rod mountedwithin a tight fitting insulating tube of PTFE (Teflon®). The winding isin four sections as shown in FIG. 10. The low voltage end of stainlesssteel secondary shown as 56D in FIG. 10 is tied to the primary HV leadon printed circuit board 56B at 56E, the high voltage terminal is shownat 56F. As a result, the transformer assumes an autotransformerconfiguration and the step-up ratio becomes 1:25 instead of 1:24. Thus,an approximately −1400 volt pulse between the + and − terminals of theinduction units will produce an approximately −35,000 volt pulse atterminal 56F on the secondary side. This single turn secondary windingdesign provides very low leakage inductance permitting extremely fastoutput rise time.

Details of Laser Chamber Electrical Components

The Cp capacitor 82 is comprised of a bank of thirty-three 0.3 nfcapacitors mounted on top of each of the MO and PA chamber pressurevessels. (Typically an ArF laser is operated with a lasing gas made upof 3.5% argon, 0.1% fluorine, and the remainder neon.) The electrodesare about 28 inches long which are separated by about 0.5 to 1.0 inchpreferably about ⅝ inch. Preferred electrodes are described below. Inthis embodiment, the top electrode is referred to as the cathode and isprovided with high voltage negative pulses in the range of about 12 KVto 20 KV for ArF lasers and the bottom electrode is connected to groundas indicated in FIG. 5A and is referred to as the anode.

Discharge Timing

By reference to FIG. 1C, Applicants have described above in detail apreferred feedback trigger control technique for timing the dischargesin the MO and the PA. In this section Applicants explain other dischargetiming issues and features.

In ArF, KrF and F₂ electric discharge lasers, the electric dischargelasts only about 50 ns (i.e., 50 billionths of a second). This dischargecreates a population inversion necessary for lasing action but theinversion only exists during the time of the discharge. Therefore, animportant requirement for an injection seeded ArF, KrF or F₂ laser is toassure that the seed beam from the master oscillator passes throughdischarge region of the power amplifier during the approximately (40 to50 billionths of a second) when the population is inverted in the lasergas so that amplification of the seed beam can occur. An importantobstacle to precise timing of the discharge is the fact that there is adelay of about 5 microseconds (i.e., 50,000 ns) between the time switch42 (as shown in FIG. 5A) is triggered to close and the beginning of thedischarge which lasts only about 40-50 ns. It takes this approximately 5microseconds time interval for the pulse to ring through the circuitbetween the C₀'s and the electrodes. This time interval variessubstantially with the magnitude of the charging voltage and with thetemperature of the saturable inductors in the circuit.

Nevertheless in preferred embodiments of the present invention describedherein, Applicants have developed electrical pulse power circuits thatprovide timing control of the discharges of the two discharge chamberswithin a relative accuracy of less than about 2 to 5 ns (i.e., 2 to 5billionths of a second). Simplified block diagrams of such circuits areshown in FIGS. 4 and 5C1.

Applicants have conducted tests which show that timing varies withcharging voltage by approximately 5-10 ns/volt. This places a stringentrequirement on the accuracy and repeatability of the high voltage powersupply charging the charging capacitors. For example, if timing controlof 5 ns is desired, with a shift sensitivity of 10 ns per volt, then theresolution accuracy of the charging circuit would be 0.5 Volts. For anominal charging voltage of 1000 V, this would require a chargingaccuracy of 0.05% which is very difficult to achieve especially when thecapacitors must be charged to those specific values at the rate of 4000times per second or greater.

Applicants' preferred solution to this problem, as described above, isto charge the charging capacitor bank of both the MO and the PA inparallel from the single resonant charger 7 as indicated in FIG. 4 andFIG. 5C1. It is also important to design the two pulsecompression/amplification circuits for the two systems so that timedelay versus charging voltage curves match as shown in FIG. 4C. This isdone most easily by using to the extent possible the same components ineach circuit.

Thus, in order to minimize timing variations (these variations arereferred to as jitter) in this preferred embodiment, Applicants havedesigned pulse power components for both discharge chambers with similarcomponents and have confirmed that the time delay versus voltage curvesdo in fact track each other as indicated in FIG. 4C. Applicants haveconfirmed that over the normal operating range of charging voltage,there is a substantial change in time delay with voltage but the changewith voltage is virtually the same for both circuits. Thus, with bothcharging capacitors charged in parallel charging voltages can be variedover a wide operating range without changing the relative timing of thedischarges.

Temperature control of electrical components in the pulse power circuitis also important since temperature variations can affect pulsecompression timing (especially temperature changes in the saturableinductors). Therefore, a design goal is to minimize temperaturevariations, a second approach is to match the pulse power components inboth the MO and the PA so that any temperature changes in one circuitwill be duplicated in the other circuit and a third approach is tomonitor temperature of the temperature sensitive components and, ifneeded, use a feedback or feedforward control to adjust the triggertiming to compensate. For typical lithography light source applications,it is not practical to avoid temperature changes, since the normal modeof operation is the burst mode described above which producessignificant temperature swings in pulse power components. Controls canbe provided with a processor programmed with a learning algorithm tomake adjustments based on historical data relating to past timingvariations with known operating histories. This historical data is thenapplied to anticipate timing changes based on the current operation ofthe laser system. Typically, adjustments for relative temperaturechanges will not be necessary during continuous operation since feedbackcontrol will automatically correct for temperature variations which aregenerally relatively slow compared to operating pulse intervals.However, correction for temperature changes could be important for thefirst pulse or the first few pulses following an idle period.

Trigger Control

The triggering of the discharge for each of the two chambers can beaccomplished separately utilizing for each circuit a trigger circuitsuch as one of those described in U.S. Pat. No. 6,016,325. Thesecircuits can add timing delays to correct for variations in chargingvoltage and temperature changes in the electrical components of thepulse power so that the time between trigger and discharge is held asconstant as feasible. As indicated above, since the two circuits arebasically the same, the variations after correction are almost equal(i.e., within about 2-5 ns of each other).

As indicated in FIGS. 6C, D, and E, performance of this preferredembodiment is greatly enhanced if the discharge in the power amplifieris timed to begin within a specific window about 2-5 ns wide andoccurring about 40 to 50 ns after the discharge in the masteroscillator. The 40 to 50 ns delay is because it takes severalnanoseconds for the laser pulse to develop in the master oscillator andanother several nanoseconds for the front part of the laser beam fromthe oscillator to reach the amplifier and because the rear end of thelaser pulse from the master oscillator is at a much narrower bandwidththan the front part. Separate trigger signals are provided to triggerswitch 46 as shown in FIG. 5A for each chamber. The actual delay ischosen to achieve desired beam quality based on actual performancecurves such as those shown in FIGS. 6C, D and E. The delay is typicallyoptimized for approximately maximum efficiency but may be optimized forother parameters. The reader should note, for example, that narrowerbandwidth and longer pulses can be obtained at the expense of pulseenergy by increasing the delay between MO trigger and PA trigger. Asshown in FIG. 6C, for maximum laser efficiency (i.e., maximum output ata given discharge voltage and given input pulse energy) the timing delayshould be within about 2 to 5 ns of optimum time delay.

Other Techniques To Control Discharge Timing

Since the relative timing of the discharges can have important effectson beam quality as indicated in the FIGS. 6C, D and E graphs, additionalsteps may be justified to control the discharge timing. For example,some modes of laser operation (specifically for example burst modeoperation) result in significant swings in charging voltage or wideswings in inductor temperature.

Monitor Timing

The timing of the discharges can be monitored on a pulse-to-pulse basisand the time difference can be used in a feedback control system toadjust timing of the trigger signals closing switch 42. Preferably, thePA discharge would be monitored using a photocell to observe dischargefluorescence (called ASE) rather than the laser pulse since very poortiming could result if no laser beam being produced in the PA. For theMO either the ASE or the seed laser pulse could be used. Voltage signalsfrom the CP capacitors 82 can also be used as feedback signals forcontrolling the relative timing of discharges for the two chambers.Preferably the clock time when the voltages crosses a selected thresholdwould be used in the feedback calculation.

Bias Voltage Adjustment

The pulse timing can be increased or decreased by adjusting the biascurrents through inductors L_(B1) L_(B2) and L_(B3) which provide biasfor inductors 48, 54 and 64 as shown in FIG. 5. Other techniques couldbe used to increase the time needed to saturate these inductors. Forexample, the core material can be mechanically separated with a veryfast responding PZT element which can be feedback controlled based on afeedback signal from a pulse timing monitor.

Adjustable Parasitic Load

An adjustable parasitic load could be added to either or both of thepulse power circuits downstream of the CO's.

Additional Feedback Control

Charging voltage and inductor temperature signals, in addition to thepulse timing monitor signals can be used in feedback controls to adjustthe bias voltage or core mechanical separation as indicated above inaddition to the adjustment of the trigger timing as described above.

Burst Type Operation

Feedback control of the timing is relatively easy and effective when thelaser is operating on a continuous basis. However, the present MOPAlaser system described herein lithography light source will normallyoperate in a burst mode such as (for example) the following to processdie spots on each of many wafers:

-   -   Off for 1 minute to move a wafer into place    -   4000 Hz for 0.2 seconds to illuminate area 1    -   Off for 0.3 seconds to move to area 2    -   4000 Hz for 0.2 seconds to illuminate area 2    -   Off for 0.3 seconds to move to area 3    -   4000 Hz for 0.2 seconds to illuminate area 3    -   4000 Hz for 0.2 seconds to illuminate area 199    -   Off for 0.3 seconds to move to area 200    -   4000 Hz for 0.2 seconds to illuminate area 200    -   Off for one minute to change wafers    -   4000 Hz for 0.2 seconds to illuminate area 1 on the next wafer,        etc.

Thus, for any laser system such as the ones described herein, sets ofdata can be obtained from calibration tests and this data can be used toprepare graphs like that shown in FIG. 6K. The data can also be used toproduce sets of bin values so that the trigger to discharge times can bedetermined based on measured temperature values and commanded chargingvoltages. It is also possible to infer inductor temperature from theoperating history of the laser. Applicants have determined that data ofthe type shown in FIG. 6K can be utilized to derive a single algorithmrelating discharge times to only two unknown: (1) charging voltage and(2) another parameter that Applicants call δ(T). With this algorithmestablished, the laser operator can merely collect a few sets of dataproviding average voltage and discharge time, and plug these numbers inthe algorithm and value for δ(T). This value of δ(T) is then insertedinto the algorithm and the algorithm then provides the discharge timemerely as a function of charging voltage. In a preferred embodiment theδ(T) values are updated automatically with a computer processorperiodically such as at 1000 pulse intervals or whenever there is asignificant change in operating conditions. In this preferred embodimentthe algorithm has the form:

$\text{MDt(V,~~δ(T))} = {\frac{\alpha}{\text{V~~+~~δ(T)v}} + \left\lbrack {\beta + {\delta\text{(T)b}}} \right\rbrack + {\gamma\left\lbrack {\text{V} + {\delta\text{(T)v}}} \right\rbrack}}$where MDt(V,δ(T)) is the discharge time for the MO and α, β, γ, ν and bare calibration constants.

This process may be repeated for many hours, but will be interruptedfrom time-to-time for periods longer than 1 minute. The length of downtimes will affect the relative timing between the pulse power systems ofthe MO and the PA and adjustment will often be required in the triggercontrol to assure that the discharge in the PA occurs when the seed beamfrom the MO is at the desired location. As shown in FIG. 1C anddescribed above, by monitoring the trigger timing and the timing oflight out from each chamber the laser operator can adjust the triggertiming (accurate to within less than 2 to 5 ns) to achieve bestperformance.

Preferably a laser control processor is programmed to monitor the timingand beam quality and adjust the timing automatically for bestperformance. Timing algorithms which develop sets of bin values foradjusting timing applicable to various sets of operating modes may beutilized in preferred embodiments of this invention. As described aboveand as shown in FIG. 6K the two most important parameters in determiningthe delay between trigger and discharge is the charging voltage and thetemperature of the saturable inductors in the pulse power system. Asindicated above, these algorithms are most useful when there is a changein the operating mode such resumption of operation after a long offperiod or if there is a substantial change in repetition rate or pulseenergy. These algorithms may be designed to switch to strict feedbackcontrol during continuous operation or burst mode operation such as thatdescribed above where the timing values for the current pulse is setbased on feedback data collected for one or more preceding pulse (suchas the immediately preceding pulse).

Preferred Technique for Jitter Control

Applicants have tested several feedback methods for jitter control.These tested methods include feedback control based on timing signalsusing peaking capacitor voltage (i.e., voltage on peaking capacitor 82for both MO and PA. The ΔT obtained by these two techniques are shown inFIG. 6J. A preferred technique based on the use of the Cp voltage is touse the time the voltage on the capacitor banks cross zero voltage asshown in FIG. 6J. For the light out techniques Applicants prefer to usethe time when the light intensity detected crosses a threshold equal toabout 10% of the typical maximum intensity.

Any combination of signals could be used for timing control. Forexample: (1) MOVcp and PA Vcp, (2) MO Vcp and PA Light Out, (3) MO LightOut and PA Vcp and (4) MO Light Out and PA Light Out. Applicants havedetermined that the fourth alternative (i.e., MO Light Out and PA LightOut) is the preferred feedback control technique, yielding the mostconsistent reliable results. Using the Vcp signals requires (for bestresults) adjustments of the ΔT values if there are significant changesin for F₂ concentration. No correction is required for F₂ concentrationchanges when both light out signals are used.

Dither to Determine Synchronization Delay

In a preferred embodiment of the present invention the timing controlfor discharge trigger for the two chambers is provided with a ditheralgorithm to assure approximately optimal timing. This improvementensures that as conditions change to the timing control continuouslysearches for the most desirable timing delay. As shown in FIG. 6C for atypical MOPA configuration, the optimum delay for maximum laserefficiency (maximum laser output for constant discharge voltage) occurswhen the time delay is about 39 ns. At ±10 ns the efficiency is down toabout 70%.

FIG. 6I is simplified block diagram model of a timing dither portion ofa preferred control system. A disturbing signal (preferably a singleperiod of sine wave of arbitrary pulse length) is generated in a “DitherGenerator” 700. This signal is added on top of the current delay commandas the laser is fired. The output energy from each pulse is measured bymonitor 702 in spectrum analysis monitor 9 as shown in FIG. 1 and fedback into dither block 700. An orthogonality integral is performed toextract the portion of the energy response due to the ditherdisturbance. As shown at 704 at the end of a complete period of thedisturbance signal, the nominal delay command in updated in accordancewith the results of the orthogonality integral.

As an example, the delay command might nominally be 35 ns. On top ofthis would be added a sine wave with 1 ns amplitude and 10 pulse period.If at the end of the ten pulses, the orthogonality integral indicatesthat the output efficiency is increased with increased delay time, thenominal delay would then be increased according to the gain setting.When at the optimal delay, the integral would yield zero, and noadjustment would be made.

Mathematically, the dither is implemented as follows:

-   1. The nominal delay command, Δt₀, is initially set to the presumed    optimal delay value based on geometry. For an N-pulse dither, the    actual delay command is the sum of the nominal command plus the    sinusoidal perturbation:

${{\Delta t}\left( \text{i} \right)} = {{{\Delta t}_{0} + {{{ɛsin}\left( \frac{2{\pi\left( {\text{i} - 1} \right)}}{\text{N}} \right)}\mspace{14mu}\text{i}}} = {1\mspace{14mu}\ldots\mspace{11mu}\text{N}}}$

-   2. The energy response, E(i), is recorded for each of the N pulses.-   3. The orthogonality integral between disturbance and response is    implemented as a discrete sum:

$\text{R} = {\underset{i = 1}{\sum\limits^{\text{N}}}{\text{E(i)sin}\left( \frac{2{\pi\left( {\text{i} - \text{1}} \right)}}{\text{N}} \right)}}$

-   4. The nominal delay command is updated based on the result of the    previous dither:    Δt₀=Δt₀+k×R

Preferably, the first dither command is always zero under thisconstruction: (choosing N=3 instead of 10 yields the 2 pulse ditherpattern that has been used in previous systems for dE/dV estimation. Theamplitude, ε, of the dither signal should be chosen so as not to standout in the output. It can be hidden below the level of the pulse-pulseenergy noise, but still extracted via the orthogonality integral. Thenominal value is not updated during the dither. It is fixed, and onlyupdated after a dither disturbance cycle has completed. A variation onstep 4 would be to use the sign of R to determine which direction tostep to move toward the maximum. The dither disturbance could be appliedcontinuously or occasionally, depending on how fast the laser operatorthinks the optimal value changes.

Feedback Timing Data Without Laser Output

Timing algorithms such as those discussed above work very well forcontinuous or regularly repeated operation. However, the accuracy of thetiming may not be good in unusual situations such as the first pulseafter the laser is off for an unusual period of time such as 5 minutes.In some situations imprecise timing for the first one or two pulses of aburst may not pose a problem. A preferred technique is to preprogram thelaser so that the discharges of the MO and the PA are intentionally outof sequence for one or two pulses so that amplification of the seed beamfrom the MO is impossible. Techniques for obtaining timing data forfeedback control without producing significant laser output arediscussed in the next two sections.

Applicants Test

Applicants have conducted careful experiments to measure the impact ofthe relative timing of the discharge of the master oscillator and thepower amplifier. These tests are summarized in FIG. 6F in which theApplicants have plotted the pulse energy (in millijoules) of amplifiedstimulated emission (ASE) from the output of the power amplifier and theline narrowed output (also in millijoules) from the MO and amplified inthe PA. Both plots are made as a function of delay between the beginningof discharge of the master oscillator and the beginning of discharge ofthe power amplifier. Beginning of discharge signals were taken fromphoto cells at the MO and PA monitoring the light output from each ofthe chambers to determine the time selected thresholds are exceeded. Thetime values plotted in FIG. 6F are displayed in FIG. 6F1. The readershould note that the energy scale of the ASE is smaller than that forthe line narrowed light output.

Lithography customer specifications call for the ASE to be a very smallfraction of the line narrowed laser output. A typical specificationcalls for the ASE to be less than 5×10⁻⁴ times the line narrowed energyfor a thirty pulse window. As is shown in FIG. 5 the ASE issubstantially zero when the narrow band pulse is maximum; i.e., in thiscase when the MO discharge precedes the PA discharge by between 25 and40 ns. Otherwise, the ASE becomes significant.

As described above, the MO and the PA pulse power circuits can betriggered with a timing accuracy of less than about 2 ns so with goodfeedback information regarding the response of the two pulse powercircuits, the MO and the PA can be discharged within the range whereline narrowed pulse energy is maximum and ASE is insignificant.Therefore, for continuous operation with good feedback control, controlof the two systems is relatively easy. However, typical operation ofthese lasers is burst mode operation as described above. Therefore, thefirst pulse of a burst could possibly produce bad results because anyfeedback data could be significantly out of date and temperature changesin the electrical components may affect their responses.

Other Techniques for Collecting Feedback Timing Data

One solution is to initiate a test pulse prior to each burst (perhapswith the laser shutter closed) so that up-to-date timing data can beobtained. This solution will typically not be preferred for severalreasons including the delay associated with closing and opening theshutter.

A better solution is the one referred to briefly above in which the twochambers are caused to discharge at relative times chosen so that therecan be no amplification of the output of the MO. From FIG. 6F we can seethat essentially zero narrowband output will result from triggering thePA earlier than about 20 ns prior to the triggering of the MO or laterthan about 70 ns after the triggering of the MO. The ASE in bothsituations is about 0.15 mJ as compared to the pulse energies of about25 mJ if the timing of the two discharges is chosen for maximum output.Applicants' preferred timing for essentially zero output discharges isto trigger the PA at least 110 ns after the trigger of the MO. Goodtargets for example, would be to trigger PA 200 ns after the MO triggeror 100 ns before the MO trigger.

In one technique if more than one minute has elapsed since the previouspulse, the PA is discharged 200 ns after the MO is discharged. Otherwisethe PA is discharged 30 to 50 ns after the MO is discharged using mostrecent feedback data as described above to produce the desired pulseenergy. The technique calls for collecting timing data, and feedbackcorrections are made for any changes in timing between trigger anddischarge. The discharge are detected by photocells detecting dischargebeyond a selected threshold produced ASE light in both the MO and the PAas indicated above. In another technique if more than one minute haselapsed since the previous pulse, the MO is discharged 40 ns after thedischarge of the PA. As before, timing data is collected and used toassure that discharges subsequent to the first pulse occur when theyshould to produce maximum or desired narrow band output and minimum ASE.

Thus, the first pulse of each burst after more than a one minute idletime produces substantially zero line narrowed output and an extremelysmall amount of ASE. Applicants estimate that the ASE for pulse windowsof at least 30 pulses, the ASE will be less than 2×10⁻⁴ of theintegrated narrow band energy. Since pulses in this preferred laser areat the rate of 4000 pulses per second, the loss of a single pulse at thebeginning of a burst of pulses is not expected to be a problem for thelaser users.

Variations

Many modifications could be made to the procedures outline above toachieve similar results. The time values such as the 30 second targetsshown of course should be chosen to provide best results. The 1 minutecould be as small as a few milliseconds so that the first pulse of eachburst is thrown away. In the first technique referred to above, the 110ns time period could be shortened to as much as about 70 ns and in thesecond technique situation the 40 ns time period could be as short asabout 20 ns. The programs could be modified to provide for two orseveral no output discharges at the start of each burst or at the startof each burst following an extended idle period. Parameters other thanthe P-cell outputs threshold could be used to determine the times ofbeginning of discharge. For example, the peaking capacitor voltage couldbe monitored. The sudden drop in voltage soon after the beginning ofdischarge could be used as the time of start of discharge.

Quick Connections

In this preferred embodiment, three of the pulse power electricalmodules utilize blind mate electrical connections so that all electricalconnections to the portions of the laser system are made merely bysliding the module into its place in the laser cabinet. These are the ACdistribution module, the power supply module and the resonant chargesmodule. In each case a male or female plug on the module mates with theopposite sex plug mounted at the back of the cabinet. In each case twoapproximately 3-inch end tapered pins on the module guide the moduleinto its precise position so that the electrical plugs properly mate.The blind mate connectors such as AMP Model No. 194242-1 arecommercially available from AMP, Inc. with offices in Harrisburg, Pa. Inthis embodiment connectors are for the various power circuits such as208 volt AC, 400 volt AC, 1000 Volt DC (power supply out and resonantcharges in) and several signal voltages. These blind mate connectionspermit these modules to be removed for servicing and replacing in a fewseconds or minutes. Lever handles such as are described in U.S. Pat. No.4,440,431 incorporated herein by reference may be used to provide goodconnection and to make removal easier. In this embodiment blind mateconnections are not used for the commutator module the output voltage ofthe module is in the range of 20 to 30,000 volts. Instead, a typicalhigh voltage connector is used.

Pulse and Dose Energy Control

Pulse energy and dose energy are preferably controlled with a feedbackcontrol system and algorithm such as that described above. The pulseenergy monitor can be at the laser as closer to the wafer in thelithography tool. Using this technique charging voltages are chosen toproduce the pulse energy desired.

Applicants have determined that this technique works very well andgreatly minimize timing jitter problems. This technique, however, doesreduce to an extent the laser operator's ability to control the MOindependently of the PA. However, there are a number of operatingparameters of the MO and the PA that can be controlled separably tooptimize performance of each unit. These other parameters include: lasergas pressure, F₂ concentration and laser gas temperature, Theseparameters preferably are controlled independently in each of the twochambers and regulated in processor controlled feedback arrangements.

Gas Control

The preferred embodiment of this invention has a gas control module asindicated in FIG. 1 and it is configured to fill each chamber withappropriate quantities of laser gas. Preferably appropriate controls andprocessor equipment is provided to inject fluorine periodically or tomaintain continuous flow or nearly continuous flow of gas into eachchamber so as to maintain laser gas concentrations constant orapproximately constant at desired levels. This may be accomplished usingtechniques such as those described in U.S. Pat. No. 6,028,880, U.S. Pat.No. 6,151,349 or U.S. Pat. No. 6,240,117 (each of which are incorporatedherein by reference). In one embodiment about 3 kP of fluorine gas(comprised of, for example, 1.0% F₂, 3.5% Ar and the rest neon for theArf laser) is added to each chamber each 10 million pulses. (at 4000 Hzcontinuous operation this would correspond to an injection eachapproximately 42 minutes.) Periodically, the laser is shut down and thegas in each chamber is evacuated and the chambers are refilled withfresh gas. Typical refills are at about 100,000,000 pulses for ArF andabout 300,000,000 for KrF.

A technique for providing substantially continuous flow of laser gasinto the chambers which Applicants call its binary fill technique is toprovide a number (such as 5) fill lines each successive line orificed topermit double the flow of the previous line with each line having a shutoff valve. The lowest flow line is orificed to permit minimumequilibrium gas flow. Almost any desired flow rate can be achieved byselecting appropriate combinations of valves to be opened. Preferably abuffer tank is provided between the orificed lines and the laser gassource which is maintained at a pressure at about twice the pressure ofthe laser chambers.

Gas injections can also be automatically made when charging voltagelevels reach predetermined values. These predetermined levels may beestablished by the performance of laser efficiency tests or they may beestablished by tests performed in the course of gas refills. For the MOthe predetermined voltage levels may be established based on bandwidthand efficiency tradeoffs.

Variable Bandwidth Control

As described above, this preferred embodiment of the present inventionproduces laser pulses much more narrow than prior art excimer laserbandwidths. In some cases, the bandwidth is more narrow than desiredgiving a focus with a very short depth of focus. In some cases, betterlithography results are obtained with a larger bandwidth. Therefore, insome cases a technique for tailoring the bandwidth will be preferred.Such techniques are described in detail in U.S. patent application Ser.Nos. 09/918,773 and 09/608,543, which are incorporated herein byreference. These techniques involves use of computer modeling todetermine a preferred bandwidth for a particular lithography results andthen to use the very fast wavelength control available with the PZTtuning mirror control shown in FIGS. 16B1 and 16B2 to quickly change thelaser wavelength during a burst of pulses to simulate a desired spectralshape. This technique is especially useful in producing relatively deepholes in integrated circuits.

Controlling Pulse Energy, Wavelength and Bandwidth

Prior art excimer lasers used for integrated circuit lithography aresubject to tight specifications on laser beam parameters. This hastypically required the measurement of pulse energy, bandwidth and centerwavelength for every pulse and feedback control of pulse energy andcenter wavelength. In prior art devices the feedback control of pulseenergy has been on a pulse-to-pulse basis, i.e., the pulse energy ofeach pulse is measured quickly enough so that the resulting data can beused in the control algorithm to control the energy of the immediatelyfollowing pulse. For a 1,000 Hz system this means the measurement andthe control for the next pulse must take less than 1/1000 second. For a4000 Hz system speeds need to be four times as fast. A technique forcontrolling center wavelength and measuring wavelength and bandwidth isdescribed in U.S. Pat. No. 5,025,455 and in U.S. Pat. No. 5,978,394.These patents are incorporated herein by reference. Additional wavemeterdetails are described in co-owned patent application Ser. No. 10/173,190which is also incorporated by reference herein.

Control of beam parameters for this preferred embodiment is alsodifferent from prior art excimer laser light source designs in that thewavelength and bandwidth of the output beam is set by conditions in themaster oscillator 10 whereas the pulse energy is mostly determined byconditions in the power amplifier 12. In a preferred embodiment,wavelength bandwidths is measured in the SAM 9. This equipment in theSAM for measuring bandwidth utilizes an etalon and a linear diode arrayas explained in the above-referenced patents and patent applications.However an etalon with a much smaller free spectral range is utilized inorder to provide much better bandwidth resolution and tracking of thebandwidth. Pulse energy is monitored in both the LAM and the SAM and mayalso be monitored at the scanner. Pulse energy may also be monitoredjust downstream of pulse stretcher 12, in each case using pulse energymonitors as described in the above patents and patent applications.These beam parameters can also be measured at other locations in thebeam train.

Feedback Control of Pulse Energy and Wavelength

Based on the measurement of pulse energy of each pulse as describedabove, the pulse energy of subsequent pulses are controlled to maintaindesired pulse energies and also desired total integrated dose of aspecified number of pulses all as described in U.S. Pat. No. 6,005,879,Pulse Energy Control for Excimer Laser which is incorporated byreference herein. The energy of each pulse in each burst is measured byphoto diode monitor 623 after pulse stretcher 12 and these measurementsare used to control pulse and dose. The rate of change of pulse energywith charging voltage is determined. A pulse energy error is determinedfor a previous pulse of the present burst. An integrated dose error isalso determined for all previous pulses in a moving pulse window (suchas the most recent 30 pulses). A charging voltage for the next pulse isdetermined using the pulse energy error, the integrated dose error, therate of change of energy with charging voltage and a reference voltage.In a preferred embodiment, the rate of change of energy with voltage isdetermined by dithering the voltage during two pulses of each burst,once lower and once higher. The reference voltage is a voltagecalculated using prior energy and voltage data. In this embodiment, themethod of determining the reference voltage during a first portion ofthe burst is different from the method used during a latter portion ofthe burst. During a first set of pulses (40 in a preferred embodiment),for each pulse, a specified voltage calculated using voltage and energydata from a corresponding pulse in a previous burst is utilized as aprediction of the voltage needed to produce a pulse energy converging ona target pulse energy. For pulses 41 and thereafter the referencevoltage for each pulse is the specified voltage for the previous pulse.

Centerline wavelength of the laser as described above may be controlledin a feedback arrangement using measured values of wavelengths at theLAM at the output of the MO and techniques known in the prior art suchas those techniques described in U.S. Pat. No. 5,978,394, WavelengthSystem for an Excimer Laser also incorporated herein by reference.Applicants have recently developed techniques for wavelength tuningwhich utilize a piezoelectric driver to provide extremely fast movementof tuning mirror. Some of these techniques are described in U.S. patentapplication Ser. No. 608,543, Bandwidth Control Technique for a Laser,filed Jun. 30, 2000 which is incorporated herein by reference. Thefollowing section provides a brief description of these techniques. Apiezoelectric stack adjusts the position of the fulcrum of the leverarm.

New Lnp With Combination Pzt-Stepper Motor Driven Tuning Mirror DetailDesign with Piezoelectric Drive

FIG. 3 is a block diagram showing features of the laser system which areimportant for controlling the wavelength and pulse energy of the outputlaser beam. In this case the wavelength is controlled by the MO so thelaser chamber shown in FIG. 3 represents the MO chamber.

Line narrowing is done by a line narrowing module 110 (designated as 10Bin FIG. 1) which contains a four prism beam expander (112a-112d), atuning mirror 114, and a grating 10C3. In order to achieve a very narrowspectrum, very high beam expansion is used in this line narrowingmodule. This beam expansion is 45× as compared to 20×-25× typically usedin prior art microlithography excimer lasers. In addition, thehorizontal size of front (116a) and back (116B) apertures are made alsosmaller, i.e., 1.6 and 1.1 mm as compared to about 3 mm and 2 mm in theprior art. The height of the beam is limited to 7 mm. All these measuresallow to reduce the bandwidth from about 0.5 pm (FWHM) to about 02 pm(FWHM). The laser output pulse energy is also reduced, from 5 mJ toabout 1 mJ. This, however, does not present a problem, because thislight will be amplified in the amplifier to get the 10 mJ desiredoutput. The reflectivity of the output coupler 118 is 30%, which isclose to that of prior art lasers.

FIG. 3A is a drawing showing detail features of a preferred wavelengthtuning technique. Large changes in the position of mirror 14 areproduced by stepper motor through a 26.5 to 1 lever arm 84. In this casea diamond pad 81 at the end of piezoelectric drive 80 is provided tocontact spherical tooling ball at the fulcrum of lever arm 84. Thecontact between the top of lever arm 84 and mirror mount 86 is providedwith a cylindrical dowel pin on the lever arm and four spherical ballbearings mounted (only two of which are shown) on the mirror mount asshown at 85. Piezoelectric drive 80 is mounted on the LNP frame withpiezoelectric mount 80A and the stepper motor is mounted to the framewith stepper motor mount 82A. Mirror 14 is mounted in a mirror mountwith a three-point mount using three aluminum spheres, only one of whichare shown in FIG. 3B. Three springs 14A apply the compressive force tohold the mirror against the spheres. Embodiments may include a bellows(which functions as a can) to isolate the piezoelectric drive from theenvironment inside the LNP. This isolation prevents UV damage to thepiezoelectric element and avoid possible contamination caused byout-gassing from the piezoelectric materials. This design has beenproven successful in correcting wavelength “chirp” which is naturallyoccurring wavelength changes occurring over time periods of about 5 to10 milliseconds during its first 30 millisecond of bursts.

Pretuning and Active Tuning

In some cases the operator of a integrated circuit lithography machinemay desire to change wavelength on a predetermined basis. In other wordsthe target center wavelength λ_(T) may not be a fixed wavelength butcould be changed as often as desired either following a predeterminedpattern or as the result of a continuously or periodically updatinglearning algorithm using early historical wavelength data or otherparameters.

Adaptive Feedforward

Preferred embodiments of the present invention includes feedforwardalgorithms. These algorithms can be coded by the laser operator based onknown burst operation patterns. Alternatively, this algorithm can beadaptive so that the laser control detects burst patterns such as thoseshown in the above charts and then revises the control parameters toprovide adjustment of mirror 14 in anticipation of wavelength shifts inorder to prevent or minimize the shifts. The adaptive feedforwardtechnique involves building a model of the chirp at a given rep rate insoftware, from data from one or more previous bursts and using the PZTstack to invert the effect of the chirp.

To properly design the chirp inversion, two pieces of information areneeded: (1) the pulse response of the PZT stack, and (2) the shape ofthe chirp. For each repetition rate, deconvolution of the chirp waveformby the pulse response of the PZT stack will yield a sequence of pulses,which, when applied to the PZT stack (with appropriate sign), willcancel the chirp. This computation can be done off line through a surveyof behavior at a set of repetition rates. The data sequences can besaved to tables indexed by pulse number and repetition rate. This tablecould be referred to during operation to pick the appropriate waveformdata to be used in adaptive feedforward inversion. It is also possible,and in fact may be preferable, to obtain the chirp shape model in almostreal-time using a few bursts of data at the start of operation each timethe repetition rate is changed. The chirp shape model, and possibly thePZT pulse response model as well, could then be updated (e.g. adapted)every N-bursts based on accumulated measured error between model anddata.

The chirp at the beginning of bursts of pulses can be controlled usingan algorithm and technique as described in U.S. patent application Ser.No. 10/012,002 which has been incorporated by reference herein.

Vibration Control

In preferred embodiments active vibration control can be applied toreduce adverse impacts resulting from chamber generated vibrations. Onesuch technique utilizes a piezoelectric load cell to monitor LNPvibrations to provide a feedback signal used to provide additionalcontrol functions to the R_(max) mirror. This technique is described inU.S. patent application Ser. No. 09/794,782 incorporated by referenceherein.

Other Bandwidth Measuring Techniques

The bandwidth of the laser beam from preferred embodiments of thepresent invention are substantially reduced compared to prior artlithography lasers. In an above section Applicants described a techniquefor utilizing an etalon having a free spectral range of about threetimes that of prior art bandwidth measuring etalons. This techniqueapproximately doubles the precision of the bandwidth measurements. Itmay be desirable to provide metrology systems for providing even greateraccuracy in bandwidth measurement than is provided by theabove-described systems. One such method is described in U.S. patentapplication Ser. No. 10/003,513 filed Oct. 31, 2001 entitled “HighResolution Etalon Grating Spectrometer”, which is incorporated byreference herein. Other high accuracy methods for measuring bandwidth,both full width half maximum and the 95% integral bandwidth can beincorporated either as a laser component or provided as test equipment.

Pulse Stretcher

Integrated circuit scanner machines comprise large lenses which aredifficult to fabricate and costs millions of dollars. These veryexpensive optical components are subject to degradation resulting frombillions of high intensity and ultraviolet pulses. Optical damage isknown to increase with increasing intensity (i.e., light power(energy/time) per cm² or mJ/ns/cm²) of the laser pulses. The typicalpulse length of the laser beam from these lasers is about 20 ns so a 5mJ beam would have a pulse power intensity of about 0.25 mJ/ns.Increasing the pulse energy to 10 mJ without changing the pulse durationwould result a doubling of the power of the pulses to about 0.5 mJ/nswhich could significantly shorten the usable lifetime of these expensiveoptical components. The Applicants have avoided this problem byincreasing substantially the pulse length from about 20 ns to more than50 ns providing a reduction in the rate of scanner optics degradation.This pulse stretching is achieved with pulse stretcher unit 12 as shownin FIG. 1. A beam splitter 16 reflects about 60 percent of the poweramplifier output beam 14B into a delay path created by four focusingmirrors 20A, 20B, 20C and 20D. The 40 percent transmitted portion ofeach pulse of beam 14B becomes a first hump of a corresponding stretchedpulse in of beam 14C. The stretched beam 14C is directed by beamsplitter 16 to mirror 20A which focuses the reflected portion to point22. The beam then expands and is reflected from mirror 20B whichconverts the expanding beam into a parallel beam and directs it tomirror 20C which again focuses the beam again at point 22. This beam isthen reflected by mirror 20D which like the 20B mirror changes theexpanding beam to a light parallel beam and directs it back to beamsplitter 16 where 60 percent of the first reflected light is reflectedperfectly in line with the first transmitted portion of this pulse inoutput beam 14C to become most of a second hump in the laser pulse. The40 percent of the reflected beam transmits beam splitter 14 and followsexactly the path of the first reflected beam producing additionalsmaller humps in the stretched pulse. The result is stretched pulse 14Cwhich is stretched in pulse length from about 20 ns to about 50 ns.

The stretched pulse shape with this embodiment has two largeapproximately equal peaks 13A and 13B with smaller diminishing peaksfollowing in time the first two peaks. The shape of the stretched pulsecan be modified by using a different beam splitter. Applicants' havedetermined that a beam splitter reflecting about 60 percent produces themaximum stretching of the pulse as measured by a parameter known as the“time integrated square” pulse length or “TIS”. Use of this parameter isa technique for determining the effective pulse duration of pulseshaving oddly shaped power vs. time curves. The TIS defined as:

$\text{t}_{IS} = {\frac{\left( {\int{\text{I(t)}{\mathbb{d}\text{t}}}} \right)^{2}}{\int{\text{I}^{2}\text{(t)}{\mathbb{d}\text{t}}}}.}$

Where I(t) is the intensity as a function of time.

In order to maintain the beam profile and divergence properties, themirrors that direct the beam through the delay propagation path mustcreate an imaging relay system that also should act as a unity,magnification, focal telescope. The reason for this is because of theintrinsic divergence of the excimer laser beam. If the beam weredirected through a delay path without being imaged, the beam would be adifferent size than the original beam when it is recombined at the beamsplitter. To create the imaging relay and afocal telescope functions ofthe pulse stretcher the mirrors are designed with a specific radius ofcurvature which is determined by the length of the delay path. Theseparation between mirrors 20A and 20D is equal to the radius ofcurvature of the concave surfaces of the mirrors and is equal to ¼ thetotal delay path.

The relative intensities of the first two peaks in the stretched pulsecan be modified with the design of the reflectivity of the beamsplitter. Also, the design of the beam splitter and therefore the outputTIS of the pulse stretcher are dependent upon the efficiency of the beamrelay system and therefore the output TIS is also subject to the amountof reflectivity of the imaging relay mirrors and the amount of loss atthe beam splitter. For an imaging relay mirror reflectivity of 97% and aloss of 2% at the beam splitter, the maximum TIS magnification occurswhen the reflectivity of the beam splitter is 63%.

The alignment of the pulse stretcher requires that two of the fourimaging relay mirrors be adjustable. Each of the two adjustable mirrorswould have tip/tilt adjustment creating a total of four degrees offreedom. It is necessary that the two adjustable mirrors be located atopposite ends of the system because of the confocal design of thesystem. To create a self-aligning pulse stretcher would requireautomated adjustment of the necessary four degrees of freedom and adiagnostic system which could provide feedback information tocharacterize the alignment. The design of such a diagnostic system,which could qualify the alignment performance, would require an imagingsystem capable of imaging both the near field and far field output ofthe pulse stretcher. By examining the overlay of the sub-pulses with theoriginal pulse at two planes (near field and far field) one would havethe necessary information to automatically adjust the mirrors to producean output where each of the sub-pulses propagate in a co-linear mannerwith the original pulse.

Relay Optics

In this preferred embodiment the output beam 14A of the masteroscillator 8 is amplified by two passes through power amplifier 10 toproduce output beam 14B. The optical components to accomplish this arecontained in three modules which Applicants have named: masteroscillator wave front engineering box, MO WEB, 24, power amplifierwavefront engineering box, PA WEB, 26 and beam reverser, BR, 28. Thesethree modules along with line narrowing module 8B and output coupler 8Aare all mounted on a single vertical optical table independent ofdischarge chamber 8C and the discharge chamber of power amplifier 10.Chamber vibrations caused by acoustic shock and fan rotation must beisolated from the optical components.

The optical components in the master oscillator line narrowing moduleand output coupler are in this embodiment substantially the same asthose of prior art lithography laser light sources referred to in thebackground section. The line narrowing module includes a three or fourprism beam expander, a very fast response tuning mirror and a gratingdisposed in Litrow configuration. The output coupler is a partiallyreflecting mirror reflecting 20 percent of the output beam for KrFsystems and about 30 percent for ArF and passing the remainder. Theoutput of master oscillator 8 is monitored in line center analysismodule, LAM, 7 and passes into the MO WEB 24. The MO WEB contains atotal internal reflection (TIR) prism and alignment components forprecisely directing the output beam 14A into the PA WEB. TIR prisms suchas the one shown in FIG. 3A can turn a laser beam 90 degrees with morethan 90 percent efficiency without need for reflective coatings whichtypically degrade under high intensity ultraviolet radiation.Alternatively, a first surface mirror with a durable high reflectioncoating could be used in place of the TIR prism.

The PA WEB 26 contains a TIR prism and alignment components (not shown)for directing laser beam 14A into a first pass through power amplifiergain medium. Alternatively, as above a first surface mirror with a highreflection coating could be substituted for the TIR prism. The beamreverser module 28 contains a two-reflection beam reversing prism relieson total internal reflection and therefore requires no optical coatings.The face where the P-polarized beam enters and exits the prism isoriented at Brewster's angle to minimize reflection lasers, making theprism almost 100% efficient.

After reversal in the beam reversing module 28, partially amplified beam14A makes another pass through the gain medium in power amplifier 10 andexits through spectral analysis module 9 and PA WEB 26 as poweramplifier output beam 14B. In this embodiment the second pass of beam14A through power amplifier 10 is precisely in line with the elongatedelectrodes within the power amplifier discharge chamber. The first passfollows a path at an angle of about 6 milliradians relative to the pathof the second pass and the first path of the first pass crosses thecenter line of the gain medium at a point half way between the two endsof the gain medium.

Beam Expansion Prisms

Coming out of the PA, the fluence of the beam is higher than anywhereelse in the system (due to small beam size and high pulse energy). Toavoid having such high fluence incident on the optical coatings in theOPuS module, where coating damage could result, beam expansion prismswere designed into the PA WEB. By expanding the horizontal beam width bya factor of 4, the fluence is reduced to ¼ its previous level.

The beam expansion is accomplished using a pair of identical prisms with20° apex angle.

The prisms are made of ArF-grade calcium fluoride and are uncoated. Byutilizing an incidence angle of 68.6° on each prism, anamorphicmagnification of 4.0 is achieved, and the nominal deviation angle of thepair is zero. The total Fresnel reflection loss from the four surfacesis about 12%.

Beam Delivery Unit

In this preferred embodiment a pulsed laser beam meeting requirementsspecified for the scanner machine 2 is furnished at the light input portof the scanner. A beam analysis module as shown at 38 in FIG. 1 called aBAM is provided at the input port of the scanner to monitor the incomingbeam and providing feedback signals to the laser control system toassure that the light provided to the scanner is at the desiredintensity, wavelength, bandwidth, and complies with all qualityrequirements such as dose and wavelength stability. Wavelength,bandwidth and pulse energy are monitored by meteorology equipment in thebeam analysis module on a pulse to pulse basis at pulse rates up to4,000 Hz using techniques described in U.S. patent application Ser. No.10/012,002 which has been incorporated herein by reference.

Other beam parameters may also be monitored at any desired frequencysince these other parameters such as polarization, profile, beam sizeand beam pointing are relatively stable, may be normally monitored muchless frequently than the wavelength, bandwidth and pulse energyparameters.

This particular BDU comprises two beam-pointing mirrors 40A and 40B oneor both of which may be controlled to provide tip and tilt correctionfor variations beam pointing. Beam pointing may be monitored in the BAMproviding feedback control of the pointing of one or both of thepointing mirrors. In a preferred embodiment piezoelectric drivers areprovided to provide pointing response of less than 7 milliseconds.

Special F₂ Laser Features

The above descriptions generally apply directly to an ArF laser systembut almost all of the features are equally applicable to KrF lasers withminor modifications which are well known in the industry. Somesignificant modifications are required, however, for the F₂ version ofthis invention. These changes could include a line selector in the placeof the LNP and/or a line selector between the two chambers or evendownstream of the power amplifier. Line selectors preferably are afamily of prisms. Transparent plates properly oriented with respect tothe beam could be used between the chambers to improve the polarizationof the output beam. A diffuser could be added between the chambers toreduce the coherence of the output beam.

Noise Reduction

Preferred embodiments include four improvements for minimizing noiseeffects in the controls for the laser system. (1) Processors areprogrammed to avoid transmittal of data on inter-module links during theapproximately 5 microseconds while the laser is firing. (2) With the CANsystem shown in FIG. 1C, cluster controllers are located at sensors andactuators and contain A to D and/or D to A converters so thattransmittal of data between modules can be in serial digital form witherror detection. The CAN equipment including device net boards isavailable from suppliers such as Woodhead Connectivity. (3) Processorscan also be programmed to avoid D to A conversion while the laser isfiring. (4) Inter-module links are shielded twisted pair conductors.

Various modifications may be made to the present invention withoutaltering its scope. Those skilled in the art will recognize many otherpossible variations.

For lithography either ArF, KrF or F₂ systems could be utilized. Thisinvention may also be applied to uses other than lithography in whichlight at ultraviolet wavelength may be needed. When the laser system isconfigured as an F₂ laser the line narrowing unit as shown at 110 inFIG. 3 would preferably be replaced with a line selection modulecomprised of one or more prisms and a total reflection mirror. Animportant improvement here is the addition of equipment to a lasersystem to deliver an ultraviolet laser beam having desire beam qualitiesto an input port of a equipment needing an ultraviolet laser lightsource. Various feedback control arrangements other than those referredto herein could be used. For laser systems including a beam deliveryunit such as 6 in FIG. 1 two actively control tilt-tip mirrors can beadded as shown with these mirrors controlled with a feedback arrangementto keep the output beam properly positioned.

The reader should understand that at extremely high pulse rates thefeedback control on pulse energy does not necessarily have to be fastenough to control the pulse energy of a particular pulse using theimmediately preceding pulse. For example, control techniques could beprovided where measured pulse energy for a particular pulse is used inthe control of the second or third following pulse. Many other laserlayout configurations other than the one shown in FIG. 1 could be used.For example, the chambers could be mounted side-by-side or with the PAon the bottom. Also, the second laser unit could be configured as aslave oscillator by including an output coupler such as a partiallyreflecting mirror. Other variations are possible. Fans other than thetangential fans could be used. This may be required at repetition ratesmuch greater than 4 kHz. The fans and the heat exchanger could belocated outside the discharge chambers.

Accordingly, the above disclosure is not intended to be limiting and thescope of the invention should be determined by the appended claims andtheir legal equivalents.

1. A two chamber high repetition rate gas discharge laser systemcomprising: A) a first laser unit comprising: 1) a first dischargechamber containing; a) a first laser gas and b) a first pair ofelongated spaced apart electrodes defining a first discharge region, 2)a first fan for producing sufficient gas velocities of said first lasergas in said first discharge region to clear from said first dischargeregion, following each pulse, substantially all discharge produced ionsprior to a next pulse when operating at a repetition rate in the rangeof 4,000 pulses per second or greater, 3) a first heat exchanger systemcapable of removing at least 16 kw of heat energy from said first lasergas, 4) a line narrowing unit for narrowing spectral bandwidths of lightpulses produced in said first discharge chamber; B) a second dischargechamber comprising: 1) a second laser gas, 2) a second pair of elongatedspaced apart electrodes defining a second discharge region 3) a secondfan for producing sufficient gas velocities of said second laser gas insaid second discharge region to clear from said second discharge region,following each pulse, substantially all discharge produced ions prior toa next pulse when operating at a repetition rate in the range of 4,000pulses per second or greater, 4) a second heat exchanger system capableof removing at least 16 kw of heat energy from said second laser gas; C)a pulse power system configured to provide electrical pulses to saidfirst pair of electrodes and to said second pair of electrodessufficient to produce laser pulses at rates of about 4,000 pulses persecond with precisely controlled pulse energies in excess of about 5 mJ;D) relay optics for directing laser beams produced in said first laserunit through said second discharge chamber to produce an amplifiedoutput beam; and E) a laser beam control system for measuring pulseenergy, wavelength and bandwidth and controlling beam quality parametersof laser beams produced by said laser system.
 2. A laser system as inclaim 1 wherein said pulse power system comprises a first pulsecompression circuit for providing high voltage electric pulses to saidfirst pair of electrodes and a second pulse compression circuit forproviding high voltage electric pulses to said second pair ofelectrodes.
 3. A laser system as in claim 2 wherein said first pulsecompression circuit comprises a first charging capacitor bank and afirst discharge switch and said second pulse compression circuitcomprises a second charging capacitor bank and a second dischargeswitch.
 4. A laser system as in claim 3 and further comprising a fistcharging means for charging said first and second charging capacitorbanks in parallel to the same or substantially the same potential inless than 250 microseconds.
 5. A laser system as in claim 4 wherein saidfast charging means is a resonant charger.
 6. A laser as in claim 3 andfurther comprising a trigger timing means for triggering said first andsecond discharge switches so as to produce electric discharges in saidfirst and second discharge regions with a relative timing accuracy ofabout 2 to 5 billionths of a second.
 7. A laser system as in claim 6wherein said trigger timing means comprises a timing and energy modulewith trigger circuits for providing trigger signals to said first andsecond discharge switches with a relative timing accuracy better than250 ps and providing light out signals representing light from saidfirst and second chambers with a relative accuracy better than 250 ps.8. A laser system as in claim 1 6 wherein said trigger timing meanscomprises a computer processor for analyzing feedback signalsrepresentative of discharges in said first chamber and in said secondchamber and for calculating trigger times for said first and seconddischarge switches so as to cause discharges in said first and secondchamber timed to produce desired quality output pulses.
 9. A laser as inclaim 7 wherein said trigger timing means comprises a processor forproducing clock pulses and a ramp voltage between clock pulses so as topermit accurate measurement of time in intervals between the clockpulses.
 10. A laser system as in claim 8 wherein said feedback signalsrepresentative of discharges comprise at least one light out time event.11. A laser system as in claim 8 wherein said feedback signalsrepresentative of discharges comprise at least two light out time event.12. A laser system as in claim 8 wherein said feedback signalsrepresentative of discharges comprise at least one time eventcorresponding to a threshold crossing of a voltage potential signalrepresenting electrical potential of a peaking capacitor bank.
 13. Alaser system as in claim 8 wherein said computer processor is programmedwith an algorithm for generating charging voltage dithers and fordetermining desired trigger timing by analyzing feedback parametersaffected by said dithers.
 14. A laser system as in claim 1 and furthercomprising a beam delivery unit and at least one tilt tip mirror formaintaining laser output beams within a desired range.
 15. A lasersystem as in claim 5 wherein said resonant charges comprises a De-Qingcircuit.
 16. A laser system as in claim 5 wherein said resonant chargescomprises a bleed-down circuit.
 17. A laser system as in claim 2 whereinsaid first and said second pulse compression circuits each compriseliquid cooled saturable indictors.
 18. A laser system as in claim 16wherein said liquid cooled saturable inductors of said first pulsecompression circuit are substantially identical to corresponding liquidcooled saturable inductors in said second pulse compression circuit. 19.A laser system as in claim 1 wherein said laser gas krypton, fluorineand a buffer gas.
 20. A laser system as in claim 1 wherein said lasergas comprises argon, fluorine and a buffer gas.
 21. A laser system as inclaim 1 wherein said laser gas comprises fluorine and said linenarrowing unit is a line selection unit.
 22. A laser system as in claim1 wherein said first and second laser units are configured as a MOPAsystem wherein said first laser unit is a master oscillator and saidsecond laser unit is a power amplifier.
 23. A laser system as in claim19 wherein said first and second laser gases comprise fluorine with saidfirst laser gas having a substantially lower fluorine concentrations ascompared to said second laser gas.
 24. A laser system as in claim 22wherein the fluorine concentration in said master oscillator iscontrolled in order to control bandwidth of said laser beams.
 25. Alaser system as in claim 21 wherein said line narrowing unit comprisesat least four beam expanding prisms, a tuning mirror and a grating. 26.A laser system as in claim 23 wherein said line narrowing furthercomprises a stepper motor and a piezoelectric driver for tuning saidtuning mirror.
 27. A laser system as in claim 23 wherein said linenarrowing unit is purged with helium.
 28. A laser system as in claim 122 wherein said power amplifier is configured for at least two beampassages through said second discharge region.
 29. A laser system as inclaim 1 and further comprising a gas control means for controllingfluorine concentration separately in said first discharge chamber and insaid second discharge chamber.
 30. A laser system as in claim 28 29wherein said gas control means is configured for continuous or almostcontinuous injections of fluorine in each discharge chamber.
 31. A lasersystem as in claim 1 wherein said laser control system comprises acontrol processing unit configured as a master control of said lasersystem.
 32. A laser system as in claim 30 wherein said master controlcomprises input ports for instructions from a lithography machine.
 33. Alaser system as in claim 1 and further comprising a control area network(CAN) having a plurality of CAN clusters.
 34. A laser system as in claim1 and also comprising a pulse stretcher for increasing pulse length oflaser pulses.
 35. A laser system as in claim 1 and further comprising aprocessor programmed to prevent transmittal of specified data duringlaser discharge.
 36. A laser system as in claim 1 35 wherein saidprocessor is also programmed to prevent A/D conversion during saiddischarges.
 37. A laser system as in claim 33 wherein said CAN isconfigured to transmit data in serial digital form with error detection.38. A laser system as in claim 1 and further comprising a processorprogrammed to discard data obtained during discharges.